Pressure sensor

ABSTRACT

A pressure sensor includes: a housing; a pressure input orifice opened at a pipe sleeve of the housing; a diaphragm sealing the pressure input orifice, the diaphragm having a first surface serving as a pressure receiving surface; a pressure sensitive unit using a direction of detecting a force as a detecting axis. A first end of the pressure sensitive unit is connected to a central area of a second surface of the diaphragm. A second end of the pressure sensitive unit is connected to the housing. The detecting axis is approximately orthogonal to the pressure receiving surface. A circumference of a portion where the central area and the first end of the pressure sensitive unit are in contact with each other is located inside a circumference of the central area. A thickness of the central area is larger than a thickness of an area surrounding the central area.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. Ser. No.12/480,098 filed Jun. 8, 2009 which claims priority to Japanese PatentApplication Nos. 2008-153254 filed Jun. 11, 2008 and 2009-027763 filedFeb. 9, 2009 all of which are expressly incorporated by reference hereinin their entirety.

BACKGROUND

1. Technical Field

The present invention relates to a pressure sensor, in particular, apressure sensor that does not use oil as a pressure receiving medium.

2. Related Art

There have been known pressure sensors using a piezoelectric resonatoras a pressure sensitive element, such as a water pressure gauge, an airgauge, and a differential pressure gauge. For example, in apiezoelectric resonator, an electrode pattern is formed on a planarpiezoelectric substrate, and a detecting axis is set in the direction ofdetecting a force. When pressure is applied to the piezoelectricresonator in the direction of the detecting axis, the resonancefrequency of the piezoelectric resonator varies. The pressure isdetected on the basis of the variation in the resonance frequency.JP-A-56-119519, JP-A-64-9331, and JP-A-2-228534 disclose pressuresensors using a piezoelectric resonator as a pressure sensitive element.When pressure is applied to a bellows via a pressure input orifice, aforce F according to the effective area of the bellows is applied to thepiezoelectric resonator as a compressive force or a tensile force via aforce transmitting unit using a pivot (a flexible hinge) as a fulcrum.Stress corresponding to the force F occurs in the piezoelectricresonator. The resonance frequency of the piezoelectric resonator variesdue to this stress. The pressure sensor measures the pressure bydetecting the variation in the resonance frequency of the piezoelectricresonator.

Hereafter, a related-art pressure sensor will be described usingexamples disclosed in JP-A-56-119519 and the like. FIG. 13 is aschematic view showing a structure of a related-art pressure sensor.

A related-art pressure sensor 501 shown in FIG. 13 includes a case 504having a first pressure input orifice 502 and a second pressure inputorifice 503 made in an opposed manner and a force transmitting member505 disposed inside the case 104. One ends of a first bellows 506 and asecond bellows 507 are connected to one end of the force transmittingmember 505 in such a manner that the one end of the force transmittingmember 505 is interposed between these bellows. The other end of thefirst bellows 506 is coupled to the first pressure input orifice 502,and the other end of the second bellows 507 is coupled to the secondpressure input orifice 503. Also, a double-ended tuning fork resonator509 serving as a pressure sensitive element is disposed between theother end of the force transmitting member 505 and an end of a substrate508 opposite to an end thereof serving as a pivot (fulcrum).

In general, a bellows of a pressure sensor is filled with a liquid sothat pressure is detected with high accuracy. As such a liquid, oilshaving high viscosity, such as a silicon oil, are typically used inorder to prevent bubbles from entering the bellows or staying in thefolds thereof.

For this reason, the first bellows 506 is filled with an oil 510 havingviscosity. If a liquid is the subject of pressure measurement, theliquid and the oil 510 make contact with and face each other via anopening 511 made at the first pressure input orifice 502. The diameterof the opening 511 is set to a size such that the oil 510 does not leakout.

In the pressure sensor 501 having the above-mentioned structure, whenthe pressure F is applied to the oil 510 filling the first bellows 506by the liquid, which is the subject of pressure measurement, thepressure F is applied to the one end of the force transmitting member505 (a swing lever supported by a pivot) via the first bellows 506. Onthe other hand, atmospheric pressure is applied to the second bellows507. Thus, a force equivalent to the atmospheric pressure is applied tothe one end of the force transmitting member 505.

As a result, a force that is equivalent to the differential pressurebetween the pressure F applied by the liquid, which is the subject ofpressure measurement, and pressure based on the atmospheric pressure isapplied to the double-ended tuning fork resonator 509 as a compressiveforce or a tensile force via the other end of the force transmittingmember 505 using the pivot of the substrate 508 as a fulcrum. Thus,stress occurs in the resonator 509 and the resonance frequency of theresonator 509 varies with the magnitude of the stress. Therefore, bymeasuring the resonance frequency, the magnitude of the pressure F isdetected.

P-A-2005-121628 presents a pressure sensor that does not include acostly force transmitting unit (cantilever) having a swing lever using apivot (flexible hinge) as a fulcrum, as used in the above-mentionedpressure sensor. In this pressure sensor, two bellows are aligned with apedestal interposed therebetween in a housing. The pressure sensor isintended to detect a pressure variation attributable to the differencebetween pressures applied to the bellows on the basis of a movement of apedestal along the direction of expansion or contraction of the bellows.Specifically, a resonator bonding pedestal is interposed between one endof a first bellows and one end of a second bellows. Both ends of apressure sensitive element are fixed to the pedestal and a housing walladjacent to the other end of the second bellows on the circumference ofthe second bellows. A reinforcing plate is disposed in such a mannerthat the reinforcing plate is symmetrical with the pressure sensitiveelement with respect to the second bellows. Both ends of the reinforcingplate are fixed to the pedestal and the housing wall.

In order to solve the problem that the bellows of the pressure sensordisclosed in JP-A-2005-121628 do not have sufficient strength againstshock applied from a direction orthogonal to the direction of a pressuredetecting axis, JP-A-2007-57395 proposes a pressure sensor where apedestal and a housing are connected in a direction orthogonal to thedirection of a pressure detecting axis using a reinforcing flexiblemember (that is, a spring).

JP-A-2006-194736 discloses a pressure sensor that is intended to detecthydraulic pressure inside an engine and is used in such a manner that itis fixed to an engine block. This pressure sensor includes a sensingunit for outputting an electrical signal according to an appliedpressure, a pressure receiving diaphragm for receiving pressure, and apressure transmitting member for transmitting the pressure from thediaphragm to the sensing unit. Specifically, a first diaphragm forreceiving pressure is provided on one end surface of a hollow metal stemand a second diaphragm for detection is provided on the other endsurface thereof. The pressure transmitting member is provided betweenthe first and second diaphragms in the stem. The pressure transmittingmember is a shaft made of metal or ceramic and is provided between thepair of diaphragms in a prestressed state. A chip with a strain gaugefunction of a pressure detection element is stuck on the outer endsurface of the second diaphragm. Pressure received by the firstdiaphragm is transmitted to the second diaphragm by the pressuretransmitting member so that the second diaphragm becomes deformed. Thedeformation of the second diaphragm is converted into an electricalsignal by the strain gauge chip. Thus, the hydraulic pressure of theengine is detected.

JP-A-2007-132697 discloses a pressure sensor that includes: a pressurereceiving diaphragm that has one surface serving as a pressure receivingsurface on which pressure to be measured acts and that becomes distortedby receiving the pressure to be measured; a pressure transmitting memberhaving one end in contact with the other surface of the pressurereceiving diaphragm; a metal stem distortion portion to which thepressure to be measured transmitted from the pressure receivingdiaphragm is applied via the pressure transmitting member; and a sensingunit that is a semiconductor chip for generating a signal on the basisof a distortion of the distortion portion. The pressure receivingdiaphragm takes a waveform that is folded multiple times. In thepressure receiving diaphragm, the central area in contact with thepressure transmitting unit is thick and the area surrounding the thickcentral area takes a waveform. Thus, the mechanical strength of thediaphragm is improved while the spring characteristic thereof isensured. The diaphragm is formed by means of stamping or cutting.

JP-A-08-159900 and JP-A-07-19981 propose specific configurations ofpressure receiving diaphragms for use in a pressure sensor.

JP-A-08-159900 describes a diaphragm having recesses and protrusions insection. A thin, flat portion is provided in the center of the pressurereceiving portion of the diaphragm. As shown in FIG. 1 ofJP-A-08-159900, the thick peripheral area of the diaphragm restrains theinfluence on the central area, of a distortion caused when welding thediaphragm to the housing. Recesses and protrusions are made on thediaphragm by half-etching both surfaces of a thin metal plate.

JP-A-07-19981 describes a pressure-resistant sensor for high temperaturewhere a pressure transmitting member is provided between the centralarea of a pressure receiving diaphragm for receiving pressure to bemeasured and a distortion sensitive element and the pressure is detectedby transmitting a distortion of the pressure receiving diaphragm to thedistortion sensitive element. In order to prevent the diaphragm fromexpanding and thus becoming deformed when the diaphragm is subjected tohigh temperature in an engine and thus causing an error in sensing, thecentral area that is originally thicker than the peripheral area of thediaphragm is recessed so that an output error attributable to avariation in the thermal distortion amount of the diaphragm caused by avariation in the temperature difference in the thickness direction ofthe diaphragm is reduced.

However, in JP-A-56-119519, JP-A-64-9331, and JP-A-2-228534, as shown bythe pressure sensor 501 of FIG. 13, the oil 510 filling the firstbellows first bellows 506 has a larger thermal expansion coefficientthan those of other elements constituting the pressure sensor 501, suchas the force transmitting member 505 and the double-ended tuning forkresonator 509. Therefore, when the temperature changes, thermaldistortions occur in the elements constituting the pressure sensor 501.Such thermal distortions act on the double-ended tuning fork resonator509 as unwanted stress, resulting in an error of a measured pressurevalue. Thus, the characteristics of the pressure sensor aredisadvantageously degraded.

Also, the oil 510 filling the first bellows 506 makes contact with andfaces a liquid, which is the subject of pressure measurement. Therefore,depending on how the pressure sensor is installed, the oil 510 may flowout toward the liquid or the liquid may flow into the first bellows 506.In this case, bubbles may occur in the oil 510. If bubbles occur in theoil 510, the oil 510 serving as a pressure transmitting medium can nolonger stably transmit a force to the double-ended tuning fork resonator509 via the force transmitting member 505. Thus, an error may occur in ameasured pressure value.

Also, as described above, the oil 510 makes contact with and faces theliquid, which is the subject of pressure measurement; therefore,depending on how the pressure sensor is installed, the oil 510 may flowout toward the liquid. Therefore, disadvantageously, the related-artpressure sensor using the oil 510 cannot be used to measure the pressureof a pure liquid into which no foreign substance must be mixed.

Also, the related-art pressure sensor 501 includes the forcetransmitting member 505 having a complicated structure. This is anobstacle to downsizing the pressure sensor. Also, the force transmittingmember 505 requires a flexible hinge having a slim constriction. Thisdisadvantageously makes the force transmitting unit 505 a costlycomponent, thereby increasing the manufacturing cost of the pressuresensor.

When the pressure sensors proposed by JP-A-2005-121628 andJP-A-2007-57395 are inclined, the bellows droop. Thus, a force appliedto the pressure sensitive element such as a double-ended tuning forkresonator varies, resulting in a variation in the resonance frequency.

Also, in the pressure sensors proposed by JP-A-2005-121628 andJP-A-2007-57395, one end of a pipe filled with an oil is connected to apressure introduction orifice and the other end of the pipe is broughtinto contact with a liquid to be measured. Therefore, as is the casewith JP-A-56-119519, JP-A-64-9331, and JP-A-02-228534, the oil fillingthe bellows and the pipe makes contact with and faces the liquid.Accordingly, depending on how the pressure sensors are installed, theoil may flow out toward the liquid or the liquid may flow into thebellows. In this case, bubbles may occur in the oil. If bubbles occur inthe oil, the oil serving as a pressure transmitting medium can no longerstably transmit a force to the double-ended tuning fork resonator viathe pedestal. This disadvantageously results in an error of the measuredpressure value.

As for JP-A-2007-57395, the pedestal interposed between the bellows issupported on the lateral surface of the housing by the reinforcingflexible member, which is a plate spring. For this reason, when thebellows move in the axis direction, a force restraining a movement ofthe pedestal works. As a result, the pressure detecting sensitivity maybe deteriorated. Also, if the hardness of the reinforcing flexiblemember is increased so that the reinforcing flexible member supports thepedestal more firmly, movements of the bellows are restrained. Thus, thepressure detecting sensitivity is disadvantageously deteriorated.

Also, in JP-A-2005-121628 and JP-A-2007-57395, the reinforcing plate isdisposed in such a manner that it is symmetrical with the pressuresensitive element with respect to the bellows, so movements of thebellows are restrained. This disadvantageously deteriorates the pressuredetecting sensitivity.

In JP-A-2006-194736 and JP-A-2007-132697, the prestressed diaphragm andcenter shaft are in contact with each other. If the diaphragm and centershaft are rigidly fixed in the pressure sensor that is used under hightemperature and high pressure, the mechanism may be broken due to thedifference in thermal expansion between these elements. Therefore, inconsideration of such thermal expansion, the diaphragm and center shafthave only a point contact with each other and are not bonded togetherusing an adhesive or the like. Therefore, there is a very highpossibility that when the diaphragm and center shaft operate due to avariation in the pressure, the contact point is misaligned. If thecontact point is misaligned, a force acting on both the diaphragm andcenter shaft leaks out. This disadvantageously prevents the pressuresensor from detecting the pressure with high accuracy.

Also, the pressure sensors described in JP-A-2006-194736 andJP-A-2007-132697 are originally used under high temperature and highpressure. For this reason, it is desirable that the force transmittingmember be as long as possible so as to secure a distance between thepressure receiving portion and the sensing unit so that thermal effecton such as a chip of the sensing unit is avoided. Therefore, it is notpreferable to apply these pressure sensors to technologies fordownsizing. Also, in JP-A-2006-194736 and JP-A-2007-132697, the centershaft is provided between the pair of diaphragms so that a force istransmitted, and the sensor chip is attached to the diaphragm in thesensing unit. For this reason, the properties of the portions adjacentto the pressure receiving portion, of the diaphragms are different fromthose of the portions adjacent to the sensing unit, thereof. This is amajor disadvantage in that the measuring accuracy cannot be increased.

As for the diaphragm described in JP-A-08-159900, the central area ofthe pressure receiving surface is thin; therefore, when an externalforce such as pressure is applied to the central area, the central areais easily damaged. For this reason, the diaphragms having a thickcentral area, described in JP-A-2007-132697 and JP-A-07-19981, areconsidered. The related-art example described in JP-A-2007-132697 doesnot have the problems that the above-mentioned JP-A-56-119519,JP-A-64-9331, and JP-A-02-228534 have; however, it has, for example, aproblem that the material of the diaphragm is limited only to metal suchas stainless steel and a problem that such a material is not suited todownsizing, since the diaphragm is formed by means of stamping. Also,the waveform diaphragm formed by means of stamping, disclosed inJP-A-2007-132697, disadvantageously requires a process of eliminatingresidual stress applied to the diaphragm during stamping, as a postprocess. Further, if, in JP-A-2007-132697, an attempt is made tomanufacture a diaphragm using the photolithography technique and etchingtechnique (hereafter collectively referred to as “photolitho-etching”)described in JP-A-08-159900, the front surface and back surface of thediaphragm are apt to be misaligned and therefore it is difficult toperform photolitho-etching with high yield, since both the diaphragmsdescribed in JP-A-2007-132697 and JP-A-08-159900 have a complicatedstructure. Also, the force transmitting unit according toJP-A-2007-132697 is point-connected to the diaphragm, so stress isconcentrated on the portion where the pressure transmitting member anddiaphragm are connected. Thus, aged deterioration is disadvantageouslyapt to occur.

As for JP-A-07-19981, the structure of the diaphragm shown in FIG. 1thereof has the above-mentioned, stamping-related problem, as inJP-A-2007-132697. Also, the structures shown in FIGS. 2 to 6 thereofhave a problem that a protection member serving as a heat shield platemust be provided on the pressure receiving surface of the diaphragm andthus the manufacturing process is complicated.

SUMMARY

An advantage of the invention is to provide a pressure sensor that usesno oil serving as a pressure receiving medium, is manufactured into asmall size with fewer steps and with high yield, and is difficult todeteriorate with time and highly sensitive.

A pressure sensor according to a first aspect of the invention includes:a housing; a pressure input orifice opened at a pipe sleeve of thehousing; a diaphragm sealing the pressure input orifice, the diaphragmhaving a first surface serving as a pressure receiving surface; apressure sensitive unit using a direction of detecting a force as adetecting axis. A first end of the pressure sensitive unit is connectedto a central area of a second surface of the diaphragm. A second end ofthe pressure sensitive unit is connected to the housing. The detectingaxis is approximately orthogonal to the pressure receiving surface. Acircumference of a portion where the central area and the first end ofthe pressure sensitive unit are in contact with each other is locatedinside a circumference of the central area. A thickness of the centralarea is larger than a thickness of an area surrounding the central area.

The above-mentioned configuration does not include oil as an element.Therefore, a problem such as oil leakage as described above does notoccur. Also, the diaphragm gives a force to the pressure sensitive unitonly in the direction of the detecting axis of the pressure sensitiveunit, so the sensitivity of the pressure sensor is improved. Also,stress is concentrated not only on the circumference of the portionwhere the pressure sensitive unit and central area are connected butalso on the circumference of the central area, so the concentration ofthe stress on the portion where the diaphragm and pressure sensitiveunit are connected is reduced. Thus, the stress is reliably istransmitted to the pressure sensitive unit. This improves thesensitivity of the diaphragm, as well as reduces aged deterioration dueto repeated concentration of stress. Also, the area surrounding thecentral area is thin and thus highly bendable, so a reduction insensitivity caused by a reduction in bendability of the whole diaphragmis prevented.

In the pressure sensor according to the first aspect of the invention,the surrounding area preferably has a flat surface and there ispreferably a step between the surrounding area and the central area.

Use of the above-mentioned configuration simplifies the manufacture ofthe diaphragm. Also, the distortion of the central area is reduced, sothe concentration of the stress on the portion where the central areaand pressure sensitive unit are connected is avoided. As a result, thedetecting accuracy is improved.

In the pressure sensor according to the first aspect of the invention,the pressure sensitive unit preferably includes a force transmittingunit having a first end in contact with a central area of the secondsurface of the diaphragm, a movable unit fixed to the force transmittingunit, and a pressure sensitive element having a first end in contactwith the fixing unit and a second end connected to the housing.

Thus, the pressure sensitive element receives a compressive force and atensile force based on a force transmitted from the movable unit in adirection identical to that of the detecting axis of the pressuresensitive element and drag transmitted from the fixing means. As aresult, the detecting efficiency of the pressure sensitive element isimproved. Also, the pressure sensitive element receives a forcetransmitted from the diaphragm via the movable unit, so the pressuresensitive element is not influenced by the installation position of thediaphragm. Thus, various types of pressure sensors according to useobjectives can be manufactured.

In the pressure sensor according to the first aspect of the invention,the pressure sensitive unit is preferably a pressure sensitive elementhaving a first end connected to the central area of the second surfaceof the diaphragm and a second end connected to the housing.

By adopting the above-mentioned configuration, an external forcereceived by the diaphragm is transmitted to the pressure sensitiveelement in a simple configuration. Also, unlike the configuration wherethe pressure sensitive unit includes a force transmitting unit, amovable unit, and a pressure sensitive element, the pressure sensitiveelement directly receives the force from the diaphragm not via the forcetransmitting unit or movable unit, so loss of the force is prevented andthe sensitivity of the pressure sensor is improved. Also, the number ofcomponents is made smaller than that in the configuration where thepressure sensitive unit includes a force transmitting unit, a movableunit, and a pressure sensitive element, and the manufacturing process issimplified. As a result, the cost is reduced.

In the pressure sensor according to the first aspect of the invention,the area surrounding the central area is preferably formed by etching.

By adopting the above-mentioned configuration, materials other thanmetals, such as a piezoelectric material, can be used as the material ofthe diaphragm, so various types of pressure sensors according to useenvironments can be constructed. Unlike stamping, photolitho-etchingfacilitates formation of a downsized diaphragm. Also, no stress isapplied to the material of the diaphragm during photolitho-etching andthere is no need to perform a process of eliminating residual stress, sothe pressure sensor is manufactured in a simplified manufacturingprocess. Also, photolitho-etching allows formation of the areasurrounding the central area in one etching process, so downsizedpressure sensors are formed without causing misalignment problems andwith high yield.

In the pressure sensor according to the first aspect of the invention,the pressure sensitive element preferably includes base portionsprovided on both ends thereof and a vibrating portion provided betweenthe base portions.

By configuring the pressure sensitive element as described above, theresonance frequency of the vibrating portion significantly varies due toa compressive force and a tensile force and the variable width of theresonance frequency is increased. As a result, a highly accuratepressure sensor having high resolution that allows detection of a smallpressure difference is formed.

A pressure sensor according to a second aspect of the inventionincludes: a housing having a pressure input orifice; a diaphragm sealingthe pressure input orifice of the housing, the diaphragm having an outersurface serving as a pressure receiving surface; a force transmittingunit connected to a central area of the diaphragm inside the housing,the force transmitting unit moving in a direction orthogonal to thepressure receiving surface of the diaphragm in conjunction with thediaphragm; and a pressure sensitive unit connected to the forcetransmitting unit and the housing, the pressure sensitive unit having adetecting axis set along an axis orthogonal to the pressure receivingsurface of the diaphragm. By forming a groove on a flat plate, thediaphragm is formed so that the central area is thicker than an areasurrounding the central area.

By adopting the above-mentioned configuration, the diaphragm makes amovement so that a force is given to the pressure sensitive unit only inthe direction of the detecting axis of the pressure sensitive unit. As aresult, the sensitivity of the pressure sensor is improved. Also, theconcentration of the stress on the portion where the diaphragm andpressure sensitive unit are connected is reduced and the stress isreliably transmitted to the pressure sensitive unit, so the sensitivityof the diaphragm is improved and aged deterioration due to repeatedconcentration of stress is reduced. Also, the area surrounding thecentral area is thin and thus highly bendable, so a reduction insensitivity caused by a reduction in bendability of the whole diaphragmis prevented. Also, the diaphragm is formed of a flat plate, so themanufacture of the diaphragm is advantageously significantly simplified.

In the pressure sensor according to the second aspect of the invention,the groove is preferably made on one surface of the diaphragm.

The above-mentioned configuration is advantageous in that a pressuresensor having good sensitivity is obtained and the manufacture of thediaphragm is facilitated.

In the pressure sensor according to the second aspect of the invention,the one surface on which the groove is made is preferably a surface towhich the force transmitting unit is connected.

By adopting the above-mentioned configuration, the concentration ofstress on the portion where the pressure sensitive unit and diaphragmare connected is reliably reduced.

In the pressure sensor according to the second aspect of the invention,the groove is preferably made on both surfaces of the diaphragm.

By adopting the above-mentioned configuration, the bending preventioneffect of the thick portion of the diaphragm is made larger than that ina case where a groove is made on one surface.

A pressure sensor according to a third aspect of the invention includes:a housing; a pair of pressure input orifices coaxially disposed onopposed end face plates of the housing; first and second diaphragms eachhaving an outer surface serving as a pressure receiving surface andsealing the pair of the pressure input orifices; a force transmittingunit connecting central areas of inner surfaces of the diaphragms insidethe housing; and a pressure sensitive element having a first endconnected to an intermediate portion of the force transmitting unit anda second end connected to the housing, the pressure sensitive elementhaving a detecting axis disposed in parallel with an axis orthogonal tothe pressure receiving surfaces of the diaphragms. By making grooves onflat plates, the diaphragms are formed so that the central areas towhich the force transmitting unit is connected are made thicker than theareas surrounding the central areas.

By adopting the above-mentioned configuration, a downsized, oilless,relative pressure sensor is obtained. Also, the concentration of stresson the portion where the pressure sensitive unit and diaphragm areconnected is reliably reduced.

A pressure sensor according to a fourth aspect of the inventionincludes: a housing; a pressure input orifice disposed at a first endface plate of the housing; a diaphragm having an outer surface servingas a pressure receiving surface, and sealing the pressure input orifice;a force transmitting unit disposed on an axis orthogonal to the pressurereceiving surface of the diaphragm inside the housing, the forcetransmitting unit being connected to a central area of an inner surfaceof the diaphragm and a second end face plate opposed to the first endface plate; and a pressure sensitive element having a first endconnected to an intermediate portion of the force transmitting unit anda second end connected to the housing, the pressure sensitive elementhaving a detecting axis set to be coaxial with an axis orthogonal to thepressure receiving surface of the diaphragm. By making a groove on aflat plate, the diaphragm is formed so that the central area to whichthe force transmitting unit is connected is made thicker than an areasurrounding the central area.

By adopting the above-mentioned configuration, a downsized, oilless,absolute pressure sensor is obtained. Also, the concentration of stresson the portion where the pressure sensitive unit and diaphragm areconnected is reliably reduced.

In the pressure sensor according to the fourth aspect of the invention,a supporting pole is preferably provided in parallel with the detectingaxis in the housing.

By adopting the above-mentioned configuration, only a force in thedetecting axis direction acts on the pressure sensitive element. As aresult, the detecting accuracy is improved.

In the pressure sensor according to the fourth aspect of the invention,the force transmitting unit is preferably a center shaft. The pressuresensitive element is preferably disposed in parallel with the centershaft.

By adopting the above-mentioned configuration, the height of the housingis reduced and thus the pressure sensor is further downsized.

In the pressure sensor according to the fourth aspect of the invention,the diaphragm is preferably fitted into a recess made on an outersurface of an end face plate of the housing so that the diaphragm isflush with the end face plate.

By adopting the above-mentioned configuration, the need to provide aprotrusion on the housing is eliminated. As a result, the height of thehousing is reduced and thus the pressure sensor is further downsized.

A pressure sensor according to a fifth aspect of the invention includes:a housing including first and second cases and a third case, the firstand second cases forming opposed end face plates, the third casesurrounding the first and second cases so that the third case forms aside face member; first and second diaphragms sealing pressure inputorifices opened at the first and second cases; a center shaft connectingcentral areas of the first and second diaphragms in the housing so thatthe center shaft and the first and second diaphragms are integral withone another so as to enable a force to be transmitted; a movablepedestal fixed to the center shaft; a fixing pedestal provided on aninner surface of the housing; a pressure sensitive element having bothends attached to the movable pedestal and the fixing pedestal, thepressure sensitive element having a detecting axis set to be in parallelwith the center shaft; and a plurality of supporting poles disposedaround the center shaft, the supporting poles connecting the first andsecond cases. By making grooves on flat plates, the diaphragms areformed so that the central areas to which the center shaft is connectedare made thicker than the areas surrounding the central areas.

By adopting the above-mentioned configuration, a downsized, oilless,relative pressure sensor is obtained. Also, only a force in thedetecting axis direction acts on the pressure sensitive element, so thedetecting accuracy is improved.

A pressure sensor according to a sixth aspect of the invention includes:a housing including first and second cases and a third case, the firstand second cases forming opposed end face plates, the third casesurrounding the first and second cases so that the third case forms aside face member; a diaphragm sealing a pressure input orifice opened atthe first case; a center shaft connected to a central area of thediaphragm in the housing so that the center shaft and the diaphragm areintegral with each other so as to enable a force to be transmitted; amovable pedestal fixed to an end of the center shaft; a fixing pedestalprovided on an inner surface of the second case; a pressure sensitiveelement having both ends attached to the movable pedestal and the fixingpedestal, the pressure sensitive element having a detecting axis set tobe coaxial with the center shaft; and a plurality of supporting polesdisposed around the center shaft, the supporting poles connecting thefirst and second cases. By making a groove on a flat plate, thediaphragm is formed so that the central area to which the center shaftis connected is made thicker than an area surrounding the central area.

By adopting the above-mentioned configuration, a downsized, oilless,absolute pressure sensor is obtained. Also, only a force in thedetecting axis direction acts on the pressure sensitive element, so thedetecting accuracy is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIGS. 1A to 1C are sectional views of a pressure sensor according to afirst embodiment of the invention.

FIG. 2 is a sectional view of a pressure sensor according to a secondembodiment of the invention.

FIG. 3 is a sectional view of a pressure sensor according to a thirdembodiment of the invention.

FIGS. 4A and 4B are drawings showing a process of manufacturing apressure sensor according to the invention.

FIG. 5 is a graph showing displacements of the central area of adiaphragm and variations in the maximum stress applied to the diaphragmwith respect to changes in width of the central area of the diaphragm.

FIG. 6 shows a pressure sensor according to a fourth embodiment of theinvention.

FIG. 7 is a cutaway perspective view of the pressure sensor according tothe fourth embodiment.

FIG. 8 is a schematic perspective view of a major element of thepressure sensor according to the fourth embodiment.

FIG. 9 shows a pressure sensor according to a fifth embodiment of theinvention.

FIG. 10 is a cutaway perspective view of the pressure sensor accordingto the fifth embodiment.

FIG. 11 is a sectional view of a pressure sensor according to a sixthembodiment of the invention.

FIG. 12 is a sectional view of a pressure sensor according to a seventhembodiment of the invention.

FIG. 13 is a schematic view showing a structure of a related-artpressure sensor.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Now, pressure sensors according to embodiments of the invention will bedescribed in detail with reference to the accompanying drawings.However, the elements, types thereof, combinations thereof, shapesthereof, relative positions thereof, and the like described in theembodiments are illustrative only unless these items are specified andthe scope of the invention is not limited to these items.

FIG. 1 is a pressure sensor according to a first embodiment of theinvention. FIG. 1A is a schematic diagram of the pressure sensor, FIG.1B is a detailed diagram of a diaphragm included in the pressure sensor,and FIG. 1C is a modification of the diaphragm. A pressure sensor 10according to the first embodiment has a cylindrical external shape andincludes a housing 12, a diaphragm 32, a center shaft 36 serving as aforce transmitting unit 34 of a pressure sensitive unit, and a pressuresensitive element 38.

Since the elements (to be described later) are housed under vacuum inthe sealed housing 12, the Q value of the pressure sensitive element 38is increased and the resonance frequency thereof is stabilized. Thus,the long-term stability of the pressure sensor 10 is ensured.

The housing 12 includes a discoid first member 14 including an upper endface plate, a discoid second member 16 including a lower end face plate,supporting poles 18 joining the first member 14 and second member 16together, a third member 20 forming a cylinder sidewall for surroundingthe side surfaces of the first member 14 and second member 16. The firstmember 14 and second member 16 have an identical diameter. The firstmember 14 and second member 16 are each provided with a pipe sleeve 22that is located on a concentric circle thereof when seen from above. Thepipe sleeves 22 protrude from the first member 14 and second member 16,respectively, and have a first pressure input orifice 24 and a secondpressure input orifice 26, respectively. A via hole 28 passing throughthe first member 14 and first pressure input orifice 24 (second member16 and second pressure input orifice 26) is made in the center of theconcentric circle.

The supporting poles 18 have a given rigidity and are formed inpositions where the first member 14 and second member 16 are opposed. Byinserting the supporting poles 18 into dowel holes (not shown) having ashape corresponding to the external shape of the supporting poles 18 soas to join the members 14 and 16 together, rigidity is obtained amongthe first member 14, second member 16, and supporting poles 18. Thisprevents unnecessary deformation of the pressure sensitive element 38(to be described later) during assembly of the pressure sensor 10 andduring use thereof. While two supporting poles are shown in the drawing,one or three or more supporting poles may be used.

In order to reduce an error made by the pressure sensor 10 due tothermal expansion, the circumference of the portion housing thesupporting poles 18 and pressure sensitive element 38, of the housing 12is preferably made of metal or ceramic having a small thermal expansioncoefficient.

Diaphragms 32 (first diaphragm 32 a, second diaphragm 32 b) are mountedon the first pressure input orifice 24 and second pressure input orifice26 so that the via hole 28 is sealed. Also, the diaphragms 32 areexposed to the outside. The diaphragms 32 become distorted in accordancewith the pressure of a liquid or gas, which is the subject ofmeasurement.

A first surface of each diaphragm 32 (first diaphragm 32 a, seconddiaphragm 32 b) is a pressure receiving surface facing the outside. Whenthe pressure receiving surface of any diaphragm 32 receives externalpressure, which is pressure to be measured, it becomes distorted. Thus,a force is applied to an end surface of the center shaft 36 that is incontact with a center area 40 of a second surface of the diaphragm 32and serves as the force transmitting unit 34 (to be described later). Asshown in FIG. 1B, each diaphragm 32 includes the thick center area 40,thin portion 42 surrounding the center area 40, and a peripheral area 44surrounding the thin portion 42, all of which are formed in a concentricmanner. The diaphragms 32 are preferably made of a highlycorrosion-resistant material such as stainless steel or metal, or may bemade of a monocrystal, such as a quartz crystal, or a non-crystalmaterial. The diaphragms 32 having such a configuration are preferablyformed by means of photolitho-etching, which generates no residualstress unlike stamping and is suitable for downsizing. In thephotolitho-etching process, steps (1) to (6) shown in FIG. 4A areperformed:

(1) A motherboard 80, which is the material of a diaphragm 32, isprepared.

(2) A positive photoresist 82 is applied to a surface of the motherboard80.

(3) The photoresist 82 is exposed to light using photomasks 84corresponding to the surface shape of the diaphragm 32.

(4) Development is performed so that a photoresist 82 a is eliminated

(5) A recess (thin portion 42 a) is made by etching the exposed area ofthe motherboard 80 to a predetermined depth.

(6) The photoresist 82 is eliminated.

Also, as shown in FIG. 1C, the thin portion 42 may be formed byhalf-etching both surfaces of a substrate. In this process, steps (1) to(6) shown in FIG. 4B are performed:

(1) A motherboard 80, which is the material of a diaphragm 32, isprepared.

(2) A positive photoresist 82 is applied to both surfaces of themotherboard 80.

(3) The photoresist 82 on both surfaces of the motherboard 80 is exposedto light using photomasks 84 corresponding to the surface shape of thediaphragm 32. In this case, the photoresist on both surfaces may beexposed simultaneously or the photoresist on one surface and then thaton another surface may be exposed.

(4) Development is performed so that an exposed photoresist 82 a iseliminated.

(5) Recesses (thin portions 42) are made on both surfaces of themotherboard 80 by etching exposed areas of the motherboard 80 to apredetermined depth.

(6) The photoresist 82 is eliminated.

Assuming that both surfaces of the motherboard 80 are etched to anidentical depth, the thin portion 42 a is formed in one etching step.Also, if the thickness of the thin portion 42 a is identical to that ofthe thin portion 42 formed by the above-mentioned single-side etching,etching is performed in half the time taken by the single-side etching.

A coating is applied to the exposed surface of the diaphragm so as toprevent corrosion of the surface by a liquid, a gas, or the like. Forexample, if a metal diaphragm is used, it may be coated with a nickelcompound. If the diaphragm is made of a piezoelectric crystal such as aquartz crystal, it is preferably coated with silicon.

The center shaft 36 is joined to the first diaphragm 32 a and seconddiaphragm 32 b in such a manner that it passes through the via hole 28.Both ends of the center shaft 36 are joined to surfaces of the centerareas 40 of the first diaphragm 32 a and second diaphragm 32 bperpendicularly. Therefore, if pressure is applied to the diaphragms 32,the center shaft 36, first diaphragm 32 a, and second diaphragm 32 b aredisplaced in an identical direction. At that time, a diaphragm 32 towhich higher pressure has been applied is displaced into the housing 12;a diaphragm 32 to which lower pressure has been applied is displaced outof the housing 12. However, the length of the center shaft 36 does notchange. Therefore, the absolute values of the inward displacement andoutward displacement are equal. Also, a movable member 36 a is fixed toa predetermined position of the center shaft 36 and is also displaced ina direction in which the center shaft 36 is displaced.

By using stainless steel, which is a material having stable strength,aluminum, ceramic, which is easy to process, or the like as the materialof the center shaft 36 in accordance with the application of thepressure sensor, a highly accurate, stable pressure sensor is formed. Inparticular, if a metal or ceramic having a small thermal expansioncoefficient is used as the material of the center shaft 36, thetemperature characteristics of the pressure sensor substantially dependon the temperature characteristics of the pressure sensitive element.Also, the center shaft 36 preferably has circular ends, and the ends areconnected to the center areas 40 in such a manner that the center shaft36 forms concentric circles with the center areas 40. This preventsconcentration of stress on a particular area of the portion where eachcenter area 40 and the corresponding end of the center shaft 36 makecontact with each other and on a particular area of the boundary betweeneach center area 40 and corresponding thin portion 42. As a result, thesensitivity of the diaphragms 32 is improved.

The pressure sensitive element 38 is made of a piezoelectric materialsuch as quartz crystal, lithium niobate, or lithium tantalite and isformed as a double-ended tuning fork resonator, a surface acoustic wave(SAW) resonator, a thickness shear resonator, or the like. Both ends ofthe pressure sensitive element 38 are connected to the movable member 36a and a fixing member 30 of the first member 14 so that the pressuresensitive element 38 is supported. The pressure sensitive element 38 hasa detecting axis set in the direction of detection of a force. Thedirection linking both ends of the pressure sensitive element 38 isparallel to the detecting axis. Also, the pressure sensitive element 38is electrically coupled to an oscillation circuit (not shown) attachedto the housing 12, and oscillates at a resonance frequency specific tothe pressure sensitive element 38 on the basis of an alternating voltagesupplied from the oscillation circuit. When the pressure sensitiveelement 38 receives extensional (tensile) stress or compressive stressfrom the movable member 36 a, the resonance frequency thereof varies. Inparticular, the resonance frequency of a double-ended tuning forkresonator element varies due to extensional stress or compressive stressto a much larger extent than those of a thickness shear resonator andthe like. Also, the variable width of the resonance frequency of adouble-ended tuning fork resonator element is larger than those of athickness shear resonator and the like. Therefore, if it is desired toobtain a pressure sensor having excellent resolution such that a smallpressure difference can be detected, a double-ended tuning forkresonator is preferably used. When a double-ended tuning fork resonatorreceives extensional stress, the amplitude width of an oscillating arm(vibration unit) thereof is reduced and thus the resonance frequency isincreased; when the double-ended tuning fork resonator receivescompressive stress, the amplitude width is increased and thus theresonance frequency is lowered. It is preferable to use a quartzcrystal, which is a material having excellent temperaturecharacteristics, as the material of a piezoelectric substrate of thedouble-ended tuning fork resonator.

In the pressure sensor having such a configuration, for example, malescrew threads are cut on the circumference of any of the pipe sleeves 22as shown in FIG. 1. Then, the pressure sensor is attached to a casecontaining a liquid or gas, which is the subject of measurement, using afitting 46 or the like serving as a female screw corresponding to themale screw. Then, the corresponding diaphragm 32 is directly broughtinto contact with the subject of measurement. Depending on the magnitudeof the pressure of a liquid or the like, which is the subject ofmeasurement, or the structure of the case, a fitting 46 having apredetermined shape or a thick fitting 46 is required.

As is understood from the above description, the pressure sensoraccording to the first embodiment does not use oil as an element.Therefore, a problem such as oil leakage does not occur. Also, the forcetransmitting unit 34 transmits a force only in the direction of the endsurfaces of the pressure sensitive element 38, so the sensitivity of thepressure sensor 10 is improved. Also, stress is concentrated not only ona circumference 401 of the portion where the force transmitting unit 34and center area 40 make contact with each other but also on acircumference 421 of the center area 40, that is, the boundary betweenthe center area 40 and thin portion 42. Thus, the concentration ofstress on the portion where the diaphragm 32 and force transmitting unit34 make contact with each other is reduced and the stress is reliablytransmitted to the force transmitting unit 34. As a result, thesensitivity of the pressure sensor 10 is improved. Also, ageddeterioration due to repeated stress concentration is reduced. Also, theportion (thin portion 42) outside the center area 40 is thin and hasgood bendability, so a pressure sensor 10 that is allowed to prevent areduction in sensitivity due to a reduction in bendability of the wholediaphragm 32 is obtained. Also, the diaphragm 32 is formed by means ofphotolitho-etching, so a downsized pressure sensor 10 can be obtained.

FIG. 2 shows a pressure sensor 50 according to a second embodiment ofthe invention. While the pressure sensor 10 according to the firstembodiment is intended to measure gauge pressure, which is indicatedusing atmospheric pressure as a zero reference, the pressure sensor 50according to the second embodiment is intended to measure an absolutepressure, which is indicated using a vacuum as a zero reference. Forthis reason, the pressure sensor 50 includes a single diaphragm 136connected to a housing 52, and a pressure sensitive unit including: acenter shaft 58 that is connected to the diaphragm 54 and serves as aforce transmitting unit 56; a movable member 58 a fixed to the centershaft 58; a fixing member 52 a fixed to the housing 52; and a pressuresensitive element 60, both ends of which are connected to the movablemember 58 a and fixing member 52 a so that the pressure sensitiveelement 60 is supported. The housing 52 is vacuum-sealed.

If the external pressure is a vacuum, the pressure applied to thediaphragm 54 is zero and the diaphragm 54 is flat. If the externalpressure is a certain level of pressure, the diaphragm 54 is displacedinto the housing 52. Accordingly, the center shaft 58 and movable member58 a are displaced. Thus, a compressive force is applied to the pressuresensitive element 60 in the direction of the end surfaces thereof.

FIG. 3 shows a pressure sensor 70 according to a third embodiment of theinvention. The pressure sensor 70 according to the third embodiment is amodification of the second embodiment. In the pressure sensor 70, apressure sensitive element constitutes a pressure sensitive unit.Specifically, a supporting portion 72 a provided at a first end of apressure sensitive element 72 is connected to a central area 76 of adiaphragm 74. A supporting part 72 b provided at a second end thereof isconnected to a fixing member 78 a fixed to a housing 78. The housing isvacuum-sealed. This eliminates the need to provide the center shaft 36and movable member 36 a according to the first embodiment or the centershaft 58 and movable member 58 a according to the second embodiment,thereby reducing the cost. Also, the pressure sensitive element 72directly receives a displacement of the diaphragm 74 as a force not viaa center shaft, so loss of the force is prevented and the sensitivity ofthe pressure sensor 70 is improved.

The inventor examined the relations between changes in width (diameter)of the central area and displacements of the central area and therelations between changes in width (diameter) of the central area andthe maximum stress (bending stress) applied to the circumference of theportion where the diaphragm and center shaft are in contact. Thediaphragm used in this examination was made of stainless steel and thethin portion was obtained by half-etching one surface of the diaphragm.The diameter of the diaphragm was 12 mm, the width of the peripheralarea thereof was 1.2 mm, the diameter of the center shaft (dispositionshown in FIG. 1B) connected to the central area was 2 mm, the thicknessof the thin portion was 100 μm, and the thicknesses of the central areaand peripheral area were 200 μm. The external pressure was atmosphericpressure. The interior (the center shaft is provided) of the housing onwhich the diaphragm was mounted was under vacuum. The width of thecentral area was changed between 2.1 mm and 8.0 mm.

FIG. 5 shows the maximum stress applied to the circumference 401 of theportion where the center area 40 and center shaft are in contact anddisplacements of the center area 40 with respect to changes in width ofthe center area 40. In FIG. 5, the lateral axis represents the width ofthe center area 40. That is, since the diameter of the diaphragm 32 isconstant, that is, 12 mm, the width of the thin portion 42 is reduced(increased) as the width (diameter) of the center area 40 is increased(reduced). The left vertical axis represents displacements of the centerarea 40 and the right vertical axis represents the maximum stressapplied to the circumference 401 of the portion where the diaphragm 32and center shaft 36 are in contact. As the displacement of the centerarea 40 is increased, the sensitivity of the diaphragm is increased.Also, as the maximum stress is reduced, the stress is also distributedto the boundary 421 between the center area 40 and thin portion 42without being concentrated only on the circumference of the contactportion. As the maximum stress is increased, the stress is concentratedon the circumference 401 of the contact portion.

As shown in FIG. 5, the maximum stress tends to be increased as thewidth of the center area 40 is reduced. In particular, when the diameterof the center area 40 becomes 3 mm or less, the maximum stress isincreased abruptly. In this case, it is conceivable that the stress isconcentrated not on the boundary but on the circumference of the contactportion.

Therefore, it is conceivable that if the diameter of the center area 40is 3 mm or more when the diameter of the center shaft is 2 mm, theconcentration of the stress on the contact portion is reduced. Note thatif the diameter of the center area 40 is 4 mm and the width of the thinportion 42 is 2.8 mm, the sensitivity of the above-mentioned diaphragmis maximized.

FIG. 6 shows a pressure sensor 100 according to a fourth embodiment ofthe invention. FIGS. 7 and 8 show a partial cutaway perspective view ofthe pressure sensor 100 and a major element perspective view thereof,respectively. The pressure sensor according to the fourth embodiment isa modification of the pressure sensor (FIG. 1A) for detecting relativepressure according to the first embodiment.

The pressure sensor 100 includes a housing 112, which is a hollowcylinder. The housing 112 is a hollow airtight case formed bysurrounding a hermetic terminal board 114 as a first member (upper endface plate), a flange end face plate 116 as a second member (lower endface plate), which are disposed separately, with a cylinder sidewall 120as a third member. A first pressure input orifice 124 and a secondpressure input orifice 126 that are recesses and communicate with theinterior of the housing are formed on the hermetic terminal board 114and flange end face plate 116, respectively. A via hole 128 that iscoaxial with the axis core of the housing 112 is made on the bottomplate of the hermetic terminal board 114 so that the interior of thehousing communicates with the outside. A first diaphragm 132 a and asecond diaphragm 132 b are fitted into the first pressure input orifice124 and second pressure input orifice 126, respectively. Thecircumferences of these diaphragms are integrally coupled to thehermetic terminal board 114 and flange end face plate 116, respectively,so that the interior of the housing is shielded from the outside. Thefirst diaphragm 132A on the hermetic terminal board 114 is intended toset atmospheric pressure and the second diaphragm 132B on the flange endface plate 116 is intended to receive pressure. As is the case with thefirst embodiment, the housing 112 having such a configuration is alsosealed and the interior thereof is maintained under vacuum using an airvent unit (not shown).

Inside the housing 112, a center shaft (a force transmitting unit) 136connecting the center areas of the inner surfaces of the first diaphragm132A and second diaphragm 132B is disposed along the axis core of thehousing 112. The center shaft 136 is joined to the diaphragms 132A and132B via the via hole 128. A movable unit 157 serving as a pressuresensitive element pedestal is integrally provided on an intermediatepart of the center shaft 136. A first end of a pressure sensitiveelement 138, which is a double-ended tuning fork resonator having adetecting axis set to be in parallel with an axis orthogonal to thepressure receiving surfaces of the diaphragms 132 (132A and 132B), isattached to the movable unit 157. A second end thereof is connected to aboss portion 139 that is provided on the hermetic terminal board 114 ofthe housing 112, projects inwardly, and serves as a pressure sensitiveelement pedestal. Thus, when the center shaft 136 moves in the axisdirection due to a pressure difference between the second diaphragm 132Bfor receiving pressure and the first diaphragm 132A for atmosphericpressure, the movable unit 157 is displaced accordingly. A forcegenerated by this displacement generates an acting force of the pressuresensitive element 138 in the detecting axis direction.

The diaphragms 132 according to the fourth embodiment are each formed bydigging a circumferential groove 142 around a central area 140, to whichthe center shaft 136 is to be connected, by performingphotolitho-etching on one surface of the motherboard 80, which is a flatplate, as shown in FIG. 4A. Thus, a step is made between the centralarea 140, to which the center shaft 136 serving as a force transmittingunit is to be connected, and the area surrounding the central area 140so that the central area 140 is thicker than the surrounding area. Inthis case, diaphragms similar to the diaphragms (FIG. 1B) according tothe first embodiment where a groove is dug into one surface are used asthe diaphragms 132. By digging the circumferential groove 142 into aflat plate, the central area 140 and a peripheral area 144 become thickportions having an identical thickness and the portion where a groove143 is made becomes a thin portion.

Inside the housing 112, multiple supporting poles 118 are disposedaround the center shaft 136 in parallel with the center shaft 136. Aswith the first embodiment, these supporting poles keep constant theinterval between the flange end face plate 116, which is the secondmember, and the hermetic terminal board 114, which is the first member,and prevent a reduction in the detecting accuracy due to deformation ofthe housing 112 caused by an external force or due to an arbitraryposture of the housing 112.

In the fourth embodiment, the hermetic terminal board 114 is used as theupper end face plate and a hermetic terminal 160 is passed through thehermetic terminal board 114 so that a signal generated by the pressuresensitive element 138 is taken out.

By adopting the fourth embodiment having such a configuration, the pairof diaphragms 132 are coupled via the center shaft 136, and the movableunit 157 provided on the intermediate portion of the center shaft 136moves (this movement is a movement attributable to the differencebetween the pressures received by the pair of diaphragms 132A and 132B)in the shaft axis direction in accordance with the movement of thediaphragms 132, together with the center shaft 136. This movementgenerates an acting force of the pressure sensitive element 138, whichis a double-sided fork tune resonator, in the detecting axis direction.As a result, there is formed a pressure sensor that uses no oil and hashigh detecting accuracy and that is downsized and easy to assemble.Also, the flange end face plate 116, hermetic terminal board 114, and acylinder sidewall 120 form the housing 112, which is a vacuum case. Thehermetic terminal board 114 is integral with the first diaphragm 132Aand the flange end face plate 116 is integral with the second diaphragm132B. Thus, the pressure sensor is easily assembled. In order to attachthe pressure sensor 100 to a case used to sink (immerse) the pressuresensor 100 into a liquid to be measured, the flange end face plate 116is surface-bonded to the case with an O ring therebetween and bolting isperformed. The O ring is disposed in such a manner that it surrounds thesecond diaphragm 132B. With regard to this attachment work, the pressuresensor according to this embodiment is not configured so that a pipesleeve including a diaphragm, to which a center shaft is connected, isscrewed to a case, unlike that according to the first embodiment. Thisprevents a failure such as one where tensile force is given to thepressure sensitive element due to the expansion of the center shaft.

Also in the fourth embodiment, the center shaft 136 and movable unit 157serving as a pedestal for fixing a pressure sensitive element may be anintegral unit obtained by cutting one member. This prevents the movableunit 157 from moving from the portion where the movable unit 157 isfixed to the center shaft 136.

In the fourth embodiment, the central areas 140 of the pair ofdiaphragms 132, to which the center shaft 136 is connected, are madethicker than the areas surrounding the central areas 140. Therefore,when pressure is received, stress is concentrated on the step betweenthe thin portion and thick portion. While the thick central area 140 isdisplaced vertically when receiving the pressure, it is bent to a lesserextent. Thus, concentration of the stress on the portion where thecenter shaft 136 and diaphragm 132 are joined together is avoided. Thisprevents an unnecessary force in a direction different from the axisdirection from acting on the center shaft 136. As a result, the pressuredetecting accuracy is improved.

Also, in the fourth embodiment, the diaphragms 132 are fitted into thefirst pressure input orifice 124 and second pressure input orifice 126,which are recesses made on the outer surfaces of the hermetic terminalboard 114 and flange end face plate 116, respectively. Therefore, thehousing 112 has no protrusion and thus the size thereof is reduced. Thisfacilitates downsizing of the pressure sensor.

Next, FIGS. 9 and 10 show a pressure sensor 200 according to a fifthembodiment of the invention. While the diaphragms 132 according to thefourth embodiment each have the circumferential groove 142 dug on onesurface thereof, diaphragms 232 (232A, 232B) according to the fifthembodiment each have a circumferential groove 242 dug on both surfacesthereof. Except for that point, the fifth embodiment is the same as thefourth embodiment, so same elements will be assigned same referencenumerals and will not be described.

In the pressure sensor 200 according to the fifth embodiment, thecircumferential groove 242 is dug into both surfaces of an areasurrounding a center area 240 of each diaphragm 232, and the center area240 and a peripheral area 244 are thick portions and the portion onwhich the circumferential groove 242 is made is relatively thin.Therefore, if each diaphragm 232 receives pressure and thus becomesdistorted, the flatness of the center area 240 thereof is maintained.Thus, a force in the axis direction is more accurately transmitted tothe center shaft 136 connected to the center area 240. As a result, thedetecting accuracy is improved. In particular, the front surface andback surface of each diaphragm 232 are symmetrical, so the diaphragms232 are very easy to assemble as components.

FIG. 11 shows a sectional view of a pressure sensor 300 according to asixth embodiment of the invention. In an example shown in the drawing,there is shown a pressure sensor that is intended to detect an absolutepressure and uses a diaphragm formed in such a manner that the centralarea, to which a center shaft is to be connected, is thicker than thearea surrounding the central area by making a circumferential groove onone surface of a flat plate. That is, the pressure sensor 300 is formedby eliminating the first diaphragm 132A for detecting atmosphericpressure from the pressure sensitive unit according to the fourthembodiment and then sealing the housing the first member only as ahermetic terminal pedestal. Also, the pressure sensor according to thefifth embodiment is different from those according to theabove-mentioned embodiments in that the center shaft and pressuresensitive element are coaxially disposed on an axis passing through thecentral area of the diaphragm for receiving pressure.

The pressure sensor 300 includes a housing 312, which is a hollowcylinder. The housing 312 is a hollow cylinder formed by using ahermetic terminal pedestal 314 as a first member (upper end face plate)and using a flange end face plate 316, which is similar to thataccording to the fourth embodiment, as a second member (lower end faceplate), and surrounding these separately disposed end face plates with acylinder sidewall 320. A pressure input orifice 326 communicating withthe interior of the housing is passed through the flange end face plate316 in such a manner that the pressure input orifice 326 is coaxial withthe axis core of the housing 312. Thus, the pressure input orifice 326forms a recess. The central portion of the recess is a via hole 328. Thediaphragm 332 is fitted into the recess so that the interior of thehousing is shielded from the outside. The diaphragm 332 is bonded to theinner wall of the pressure input orifice 326 so that it is integrallycoupled to the flange end face plate 316. The diaphragm 332 is intendedto receive the pressure of a liquid to be measured. The hermeticterminal board 314 is formed as an end face plate where none of apressure input orifice and a diaphragm is provided. As with the otherembodiments, the housing 312 having such a configuration is also sealedand the interior thereof is maintained under vacuum using an air ventunit (not shown).

Inside the housing 312, a center shaft (a force transmitting unit) 336is vertically provided on a center area 340 of the inner surface of thediaphragm 332 along the core axis of the housing 312. A movable unit 357serving as a pressure sensitive element pedestal is integrally providedat the tip of the center shaft 336. A first end of a pressure sensitiveelement 338, which is a double-ended tuning fork resonator having adetecting axis coaxial with the center shaft 336, is attached to themovable unit 357. A second end thereof is connected to a pedestal 315that is provided on the central area of the hermetic terminal board 314of the housing 312 and projects inwardly. Thus, when the diaphragm 332for receiving pressure receives pressure and thus becomes distorted, thecenter shaft 336 moves in the axis direction. This movement generates anacting force of the pressure sensitive element 338 connected to themovable unit 357 in the detecting axis direction.

As shown in FIG. 4A, the diaphragm 332, to which the center shaft 336 isto be connected, is formed by digging a circumferential groove 342 intothe area surrounding the central area 340, to which the center shaft 336is to be connected, by performing photolitho-etching on one surface ofthe motherboard 80, which is a flat plate. Thus, the central area 340,to which the center shaft 336 serving as a force transmitting unit is tobe connected, is formed in such a manner that it becomes thicker thanthe area surrounding the center shaft 336 by a step. A diaphragm similarto the diaphragm (FIG. 1B) according to the first embodiment where agroove is dug into one surface is used as the diaphragm 332. By diggingthe circumferential groove 342 into a flat plate, the central area 340and a peripheral area 344 become thick portions having an identicalthickness and the portion where a groove 343 is made becomes a thinportion.

Also, in the above-mentioned housing 312, multiple supporting poles 318are disposed around the center shaft 336 in parallel with the centershaft 336. As with the other embodiments, these supporting poles keepconstant the interval between the flange end face plate 316, which isthe second member, and the hermetic terminal board 314, which is thefirst member, and prevent a reduction in the detecting accuracy due todeformation of the housing 312 caused by an external force or due to anarbitrary posture of the housing 312.

Also in the sixth embodiment, the hermetic terminal board 314 is used asthe upper end face plate and a hermetic terminal (not shown) is passedthrough the hermetic terminal board 314 so that a signal generated bythe pressure sensitive element 338 is taken out, as in the fourthembodiment.

By adopting the sixth embodiment having such a configuration, the flangeend face plate 316, hermetic terminal pedestal 314, and cylindersidewall 320 form the housing 312, which is a vacuum case, and theflange end face plate 316 and diaphragm 332 are integral with eachother. Thus, the pressure sensor is easily assembled. The diaphragm 332for receiving pressure and center shaft 336 are connected concentricallyin a line. The movable unit 357 provided at the tip of the center shaft336 moves in the shaft axis direction in accordance with a movement ofthe diaphragm 332. This movement generates an acting force of a pressuresensitive element 338, which is a double-sided fork tune resonator, inthe detecting axis direction. Therefore, a pressure sensor that uses nooil and has high detecting accuracy and is small and easy to assemble isformed.

Also, in the sixth embodiment, the center shaft 336 and the movable unit357 serving as a pedestal for fixing a pressure sensitive element may bean integral unit obtained by cutting one member. This prevents themovable unit 357 from moving from the portion where the movable unit 357is fixed to the center shaft 336.

FIG. 12 shows a pressure sensor 400 according to a seventh embodiment ofthe invention. While the diaphragm 332 according to the sixth embodimenthas the circumferential groove 342 dug on one surface thereof, adiaphragm 432 according to the seventh embodiment has circumferentialgrooves 442 dug on both surfaces thereof. Except for that point, theseventh embodiment is the same as the sixth embodiment, so same elementswill be assigned same reference numerals and will not be described.

In the pressure sensor 400 according to the seventh embodiment, thecircumferential groove 442 is dug into both surfaces of an areasurrounding a center area 440 of the diaphragm 432, and the center area440 and a peripheral area 444 are thick portions and the portion onwhich the circumferential grooves 442 are made are relatively thin.Therefore, even if the diaphragm 432 receives pressure and thus becomesdistorted, the flatness of the center area 440 thereof is maintained.Thus, a force in the axis direction is more accurately transmitted tothe center shaft 336 connected to the center area 440. As a result, thedetecting accuracy is improved. In particular, the front surface andback surface of the diaphragm 432 are symmetrical, so the diaphragm 432is easy to assemble as a component.

1. A pressure sensor comprising: a housing; a pair of pressure inputorifices coaxially disposed on opposed end face plates of the housing;first and second diaphragms each having an outer surface serving as apressure receiving surface and sealing the pair of the pressure inputorifices; a force transmitting unit connecting central areas of innersurfaces of the diaphragms inside the housing; and a pressure sensitiveelement having a first end connected to an intermediate portion of theforce transmitting unit and a second end connected to the housing, thepressure sensitive element having a detecting axis disposed in parallelwith an axis orthogonal to the pressure receiving surfaces of thediaphragms, wherein by making grooves on flat plates, the diaphragms areformed so that the central areas to which the force transmitting unit isconnected are made thicker than areas surrounding the central areas, andthe groove is made on both surfaces of the diaphragm.
 2. The pressuresensor according to claim 1, wherein the force transmitting unit is acenter shaft, and the pressure sensitive element is disposed in parallelwith the center shaft.
 3. The pressure sensor according to claim 1 or 2,wherein the area surrounding the central area is formed by etching. 4.The pressure sensor according to claim 1 or 2, wherein the pressuresensitive element includes: base portions provided on both endsthereof', and a vibrating portion provided between the base portions. 5.The pressure sensor according to claim 3, wherein the pressure sensitiveelement includes: base portions provided on both ends thereof; and avibrating portion provided between the base portions.
 6. A pressuresensor comprising: a housing; a pressure input orifice disposed at afirst end face plate of the housing; a diaphragm having an outer surfaceserving as a pressure receiving surface, and sealing the pressure inputorifice; a force transmitting unit disposed on an axis orthogonal to thepressure receiving surface of the diaphragm inside the housing, theforce transmitting unit being connected to a central area of an innersurface of the diaphragm and a second end face plate opposed to thefirst end face plate; and a pressure sensitive element having a firstend connected to an intermediate portion of the force transmitting unitand a second end connected to the housing, the pressure sensitiveelement having a detecting axis set to be coaxial with an axisorthogonal to the pressure receiving surface of the diaphragm, whereinby making a groove on a flat plate, the diaphragm is formed so that thecentral area to which the force transmitting unit is connected is madethicker than an area surrounding the central area, and the groove ismade on both surfaces of the diaphragm.
 7. The pressure sensor accordingto claim 6, wherein the force transmitting unit is a center shaft, andthe pressure sensitive element is disposed in parallel with the centershaft.
 8. The pressure sensor according to claim 6 or 7, wherein thearea surrounding the central area is formed by etching.
 9. The pressuresensor according to claim 6 or 7, wherein the pressure sensitive elementincludes: base portions provided on both ends thereof; and a vibratingportion provided between the base portions.
 10. The pressure sensoraccording to claim 8, wherein the pressure sensitive element includes:base portions provided on both ends thereof; and a vibrating portionprovided between the base portions.
 11. A pressure sensor comprising: ahousing including first and second cases and a third case, the first andsecond cases forming opposed end face plates, the third case surroundingthe first and second cases so that the third case forms a side facemember; a diaphragm sealing a pressure input orifice opened at the firstcase; a center shaft connected to a central area of the diaphragm in thehousing so that the center shaft and the diaphragm are integral witheach other so as to enable a force to be transmitted; a moveablepedestal fixed to an end of the center shaft; a fixing pedestal providedon an inner surface of the second case; a pressure sensitive elementhaving both ends attached to the movable pedestal and the fixingpedestal, the pressure sensitive element having a detecting axis set tobe coaxial with the center shaft; and a plurality of supporting polesdisposed around the center shaft, the supporting poles connecting thefirst and second cases, wherein by making a groove on a flat plate, thediaphragm is formed so that the central area to which the center shaftis connected is made thicker than an area surrounding the central area.12. A pressure sensor comprising: a housing having a pressure inputorifice; a diaphragm sealing the pressure input orifice in the housing,the diaphragm having an outer surface serving as a pressure receivingsurface; a force transmitting unit connected to a central area of thediaphragm inside the housing, the force transmitting unit moving in adirection orthogonal to the pressure receiving surface of the diaphragmin conjunction with the diaphragm; a pressure sensitive elementconnected to the force transmitting unit and the housing, the pressuresensitive element having a detecting axis set along an axis orthogonalto the pressure receiving surface of the diaphragm, wherein by forming agroove on a flat plate, the diaphragm is formed so that the central areais thicker than an area surrounding the central area, and the groove ismade on both surfaces of the diaphragm.
 13. The pressure sensoraccording to claim 12, wherein the force transmitting unit is a centershaft, and the pressure sensitive element is disposed in parallel withthe center shaft.
 14. The pressure sensor according to claim 12 or 13,wherein the area surrounding the central area is formed by etching.